Percussion tone generating device



SPEAKER R. O. BARBER 2 Sheets-$heet l AMPLIFIER TIMING nd SWITCHINGTIMING and SWlTCHING Ronald 0. Barber BY wwzuwm Feb. 2 8, 1967PERCUSSION-TONE GENERATING DEVICE Filed March 23, 1964 Q KEYER KEYERTONE GEN. l

TONE

PERCUSSION TONE GENERATING DEVICE Filed March 23, 1964 2 Sheets-Sheet 2INVENTOR Ronald 0. Barber BY flammww ww Affws United States Patent()fitice 3,306,969 PERCUSSIDN TGNE GENERATING DEVICE Ronald 0. Barber,Fort Wayne, Ind, assignor to The Magnavox Company, Fort Wayne, Ind, acorporation of Indiana Filed Mar. 2.3, 1964, Ser. No. 353,905 12(Ilaims. (Cl. 84-124) This invention relates generally to electronicmusical instruments, and more particularly to a circuit system wherebythe tone producing means of a key or buttonoperated electronic musicalinstrument can produce an output simulating that produced by a piano orother percussion instrument.

Heretofore, various efforts have been made to cause an electricalmusical instrument to simulate the sounds of a variety of musicalinstruments. These efforts have achieved various degrees of success buthave not been entirely satisfactory when directed to the simulation ofsounds produced by percussion types of musical instruments, particularlypianos.

It is a general object of the present invention to provide an improvedcircuit for use in electrical musical instruments to simulate the soundsproduced by percussion types of musical instruments.

A further object is to provide a circuit operable by the usual keys of akey-operated electrical musical instrument and responsive to keyactuation to control the tone producing means of the instrument andsimulate the sound produced by a piano.

A further object is to provide a system which not only responds to keyactuation itself but to the velocity of actuation of keys and the mannerof their operation.

Described briefly, a typical embodiment of the present inventionincorporates a system of four timing circuits for each playing key of amanual of an electronic organ. The circuits for each key are activatedby switching functions effected by actuation of the key and by actuationof the sustain pedal of the organ.

The switching functions are accomplished in a predetermined sequence ofswitch events as the key is actuated, to provide a control voltageoutput whose amplitude-time characteristic is analogous to that of thesound which would be produced by striking the key of a piano in the sameway. The output voltage is used to control the tone signal output of thetone generator producing the tone signal output for the key, to producea sound output much like that of a piano.

The full nature of the invention will be understood from theaccompanying drawings and the following description and claims.

FIG. 1 is a schematic diagram of the circuit of the present invention.

FIG. 2 is an example of a waveform produced by the circuit of theillustrated embodiment, when used to simulate a piano sound.

FIG. 3 is a mechanical schematic diagram illustrating a typical keyswitch mechanism for achieving a sequence of switching functions by thenormal operation of a key of an electronic musical instrument, in orderto control the circuit of the present invention to obtain the desiredoutput.

Referring to the drawings in detail, and particularly FIG. 1, thereof,several note playing keys 6 of an electrical musical instrument areindicated, together with a tone generator 7 and keyer 8 for each, andthe timing circuitry and switches 9 of the present invention for each.Switch contacts 11, 12, 13, and 14 are shown. A negative direct currentpotential is normally applied at all times to the contact 11 by a bussconnected to the terminal 16. An example of the level of potential isvolts below ground. A positive direct current potential is nor-3,306,969 Patented Feb. 28, 1967 mally applied from terminal 18 to themovable contact 17 which is normally closed with the fixed contact 14,these being contacts of a pedal sustain switch. Fifteen volts positivepotential is normally applied to terminal 18. Both of these sources ofpotential are of the low impedance type. Reference numeral 19 designatesground in the circuit.

A direct current voltage is provided on output conductor 21 of thecircuit of this invention as a result of the operation of the circuit,and is of the form shown in FIG. 2. This voltage is applied to a musicsignal source such as provided by the tone generator 7 and keyer 8, forexample. The music signal output of the tone generator is provided atterminal 24 and its amplitude follows the characteristic of the controlvoltage on the conductor 21. It may be coupled on conductor 23 to outputamplifier 25 for amplification and then translation to sound at thespeaker.

A resistor 26 and capacitor 27 in parallel are connected between supplyterminal 18 and the junction 28 which is connected to the movablecontactor 29 which is engageable alternatively with the switch contactterminals 11 and 12. Terminal 12 is connected through the resistance 31and diode 32 and capacitor 33 in series to ground. The diode 32 isoriented such as would provide low resistance therein to the flow ofpositive current in a direction from ground through diode 32 to terminal12.

Capacitor 34 and resistor 36 are connected in parallel with resistor 37,this parallel arrangement being connected between the output conductor21 and the junction 38 of the capacitor 33 and diode 32.

A movable contactor 39 is normally closed with the fixed contact 13 andis connected through the diode 41 and resistor 42 in series to thejunction 38. Diode 41 is oriented such as to provide low resistancetherein to the flow of positive current in a direction from contactor 39to resistance 42.

Referring now to FIG. 3, the various parts corresponding to contactterminals and contactors in FIG. 1 are assigned the same referencecharacters.

For each note playing key, a switch actuator arm 46 is coupled to thekey, the actuator arm being pivotally mounted at 47 to a stationaryframe 48. A contact wire support block 49 is afiixed to the frame 48 andsupports three spring contact wires 12, 29, and 39 in cantileverfashion. Wire 12 normally extends straight out from the block and has adownturned free outer end 12a. The outer ends of wires 29 and 39 arereceived in apertures 52 and 53 in the nonconductive downturned leg 51of the actuator arm.

When the key is not actuated, the return spring 55 holds the actuatorarm down so that wire 29 normally engages a buss contact 11 and wire 39normally engages buss contact 13. These wires are resilient and highlyconductive.

When the key coupled to the actuator arm is depressed, and the leg 51moves upwardly, the outer ends of wires 29 and 39 are raised by the armto break contact thereof with the contacts 11 and 13. Then upon releaseof the key, the return spring will return the arm and re-engage wires 29and 39 to engagement with the contacts 11 and 13, respectively.

It is desirable that upon depressing a note playing key coupled to theactuator arm 46 pivoting at 47, a certain sequence of switching eventstakes place. The first event is that the contact wire or movablecontactor 39, which is normally closed with the buss contact 13, openswhen the key is depressed approximately one-half the full key travel.Then when the key reaches approximately threequarters depression, thecontact wire or movable contactor 29 which is normally closed with thebuss contact 11 opens. The opening of contacts 29 and 11 after theopening of contacts 39 and 13 can be obtained by proper 3 location ofthe apertures 52 and 53, the wires 29 and 39, and the contacts 11 and13.

Then when the key reaches full depression, the movable contactor 29closes with the contact wire 12. If the key is held in this position,the condition of the switches will remain unchanged.

Then when the key is released, as the first event, the movable contactor29 moves away and breaks from contact 12. Then at approximatelyone-quarter of the distance from full key depression, the movablecontactor 29 closes with the fixed contact 11. Then when the key risesto approximately one-half full depression, the movable contactor 39again closes With the fixed contact 13.

During all of this time, the movable contactor 17 remains closed withthe fixed contact 14, this movable contactor 17 opens only when thesustain pedal 43 is depressed, and it closes immediately when thesustain pedal is released.

It has been stated that there are four basic timing circuits which shapethe output of the circuit of the present invention. These are: thevelocity timing circuit, the impulse timing circuit, the decaytimingcircuit, and the release timing circuit. Each of these circuitswill be best understood when related to the operation of the invention.

Velocity timing circuit The velocity timing circuit consists of theresistance 26 and capacitor 27 parallel, these being normally connectedin series between supply terminal 18 and the source of negativepotential 16 through the movable and fixed contacts 29 and 11,respectively, of the normally closed switch in series therewith. Asmentioned previously, this first normally closed switch opens when thekey is depressed approximately three-quarters. While the switch isclosed, capacitor 27 is charged to the potential difference between thesupply at 16 and that at 18, this being normally 35 volts. As the key isdepressed from three-quarters travel to its full extent of depression,capacitor 27 discharges through resistance 26. Accordingly, themagnitude of voltage remaining on capacitor 27 when the key reaches fulldepression and closes the movable contactor 29 with contactor 12, isproportional to the time required to move the key from three-quarters tofull position. This means that the voltage remaining on capacitor 27 isproportional to the velocity of key movement.

Impulse timing circuit Initially, before the key was depressed,capacitor 33 had charged to the positive potential of terminal 18. Thecontrol voltage Ep at point 72 was of a positive value (point 55 oncurve, FIG. 2) approximately equal to that of terminal 18. This voltage,being positive, back biased the keyer diode 56 so that no signalappeared at the tone signal output terminal 24.

As the key was depressed, it first broke the contact between damperswitch contacts 13 and 39. Then, when the movable contactor 29 closeswith the contactor 12 as the key reaches its full depression, thepositive charge on capacitor 33 is discharged through the diode 32 andresistor 31, and capacitor 33 begins (time t=0, point 61 on the curve ofFIG. 2) to be charged negatively to approximately the same voltage asremains on capacitor 27, the latter capacitor being large compared tocapacitor 33. Resistor 31 determines the rapidity of the chargingaction.

As capacitor 33 charges negatively and reaches a potential more negativethan the instantaneous potential existing at point 68, diode 56 becomesforward biased and the tone signal is coupled to point 24. At the sametime, current flows either through resistance 66 and transistor 67 orthrough resistance 71, then through diode 56 and resistance 37 andbecomes part of the discharge current for capacitance 27. As thepotential on capacitance 33 becomes more negative, the signal coupled topoint 24 increases in amplitude. Maximum signal is reached when thenegative potential on capacitance 33 is more negative than the peaknegative swing at point 63. For

example, if transistor 67 were operated as a switch by driving itrapidly from saturation to cut-oif, the peak negative voltage at point68 would equal the negative potential at point 69 and maximum outputwould occur when the potential on capacitance 33 was more negative thanthe potential at point 69.

As the voltage on capacitor 33 approaches the voltage on capacitor 27,the diode 32 begins to turn off and the RC time constant of thevelocitytiming circuit has no further elfect on the circuit action. Then (timei=1, point 62, FIG. 2) diode 32 has turned off, allowing capacitor 27 todischarge without disturbing the charge on capacitor 33. Capacitor 33begins to discharge through resistor 37, and to charge capacitor 34through resistor 36.

Initially, as capacitor 33 begins to discharge, it sees a low impedancepath formed by resistor 36, capacitor 34, keyer diode 56, transistor 67,and resistor 66. Of course, resistance 37 in parallel with resistor 36and capacitor 34 contributes somewhat to the low impedance.Consequently, capacitor 33 discharges quickly at first until it hascharged up capacitor 34. The effect of this rapid discharge is that thecontrol voltage output of. the circuit on conductor 21 falls rapidlyfrom point 62 to points 63 on the curve of FIG. 2. Thus, it is seen thatthe impulse timing circuitry produces a control voltage impulse for thesignal source, this impulse being of the character represented betweenpoints 61 and 63 on the curve in FIG. 2. The tone signal outputamplitude is controlled accordingly.

Decay timing circuit After capacitor 34 charges up (time t:2, point 63,FIG. 2), the low impedance charging path through capacitor 34 inparallel with resistance 37 is effectively terminated. Therefore, theimpedance to charging current for capacitor 33 then rises sharply.Because resistance to charging current in the keyer is low by comparisonwith resistance 37, the decay rate for the circuit output voltage onconductor 21 is determined primarily by resistance 37 and the voltageslowly drops. The effect is shown on the curve of FIG. 2 from point 63to point 64. The keyer diode 56 will remain forward biased and continueto conduct tone signals to the output terminal 24. The amplitude thereofwill decay according to the decay of the control voltage shown on FIG.2.

Release timing circuit Normally, when a piano key is released, a damperis applied to the piano strings. The same effect is obtained in thecircuit of the present invention.

To achieve this effect, it is desired to stop the signal before it hascompleted its normal decay through resistance 37. For this purpose, theswitch incorporating movable contactor 39 and fixed contactor 13 isemployed. This switch is normally closed when the key is in theundepressed condition. The movable contactor 39 moves away from thefixed contactor 13 when the key is depressed approximately one-half itsfull travel. Thus, the switch is open at that time. The switch remainsopen during the remainder of the key travel and will continue to remainopen until the key again returns to approximately one-half its fulltravel. Thus, it is seen that when the key is released, the switchcontactor 39 will close with the contactor 13 when the key rises toapproximately one-half of travel. Closure of these contacts (time t=3,point 64, FIG. 2) provides a discharge path for capacitor 33 through theresistance 42, diode 41, switch contacts 39 and 13, and the normallyclosed sustain switch contacts 17 and 14 to the source of positivepotential at terminal 18. This results in a quick discharge of capacitor33 upon release of the key, which simulates the action that takes placewhen the damper in a piano is applied to the piano strings upon releaseof the piano key. The effect on the control signal is shown in FIG. 2be' tween points 64 and 65. The effect on the tone signal output atterminal 24 is the same and keyer diode 56 again becomes back biased.

Sustain pedal The sustain pedal action of a piano is also simulated withthe circuit of the present invention. As mentioned previously, theswitch contacts 14 and 17 are used to provide the sustaining action.When these contacts are closed, the release timing circuit provides forrapid discharge of capacitor 33 upon release of the key, as previouslydescribed. .However, when the sustain pedal 43 is depressed, the sustainswitch is opened. This opens the release circuit to disable it, and thecapacitor 33 must continue to discharge through resistances 37 and 66even though the key is released. Consequently, the release portion ofthe curve will continue as indicated by the dotted outline 76 and thetone output of the instrument is sustained. One sustain switch servesthe circuits for all keys, according to this invention.

As indicated above, a keyswitch mechanism such as shown in FIG. 3, orone useful to obtain the same sequence of switching events is providedfor each noteplaying key. However, the contacts 11 and 13 may each be abuss b-ar common to the keyswitch mechanism for all of the keys.Interaction between notes while the sustain switch is open is preventedby the diode 41. In the absence of diode 41 in the circuit for each key,a negative voltage appearing on capacitor 33 at junction 38 for one ormore keys might cause current flow through the buss b-ar connected tocontact 13 and therefore, through resistances 42 of the circuits forother keys, which could have the effect of keying action on another tonegenerator. This action is avoided by use of a diode 41 in each of thecircuits according to this invention.

Examples of component values useful in circuitry according to theillustrated embodiment of this invention are as follows:

Potential at terminal 16, volts D.C. 20 Potential at terminal 1%, voltsD.C. +15 Potential at terminal 69, volts 11C. l8

Potential at ground 19, volts D.C

Resistors '70, 71, ohms 10,000 Resistor 66, ohms 4'70 Resistor 37,megohms 2.2 Resistor 36, ohms 680,000 Resistor 42, ohms 390,000 Resistor31, ohms 3,300 Resistor 25, ohms 15,000 Capacitor 34,, mid. 0.1Capacitor 33, mfd. 0.47

Timing sequence The release phase is that beginning with release of thekey from the stop condition until the key returns to its initialposition. Its events are:

(l) Contact 29 breaks from 12. (2) Contact 29 contacts 11. (3) Contact39 contacts 13.

While the invention has been disclosed and described in some detail inthe drawings and foregoing description, they are to be considered asillustrative and not restrictive in character, as other modificationsmay readily suggest themselves to persons skilled in this art and withinthe broad scope of the invention, reference being had to the appendedclaims.

The invention claimed is:

1. In an electrical musical instrument, the combination comprising:

a tone signal source having a tone signal output terminal;

a plurality of keys;

a plurality of switches;

means coupling said plurality of switches to one of said keys wherebyswitches of said plurality are operable in sequence by actuation of oneof said keys;

first capacitance means coupled through a first switch of said pluralityto a first terminal at substantially constant negative potential tocharge said first capacitance means;

second capacitance means coupled through a second switch of saidplurality of switches to a source of positive potential to establish aninitial positive charge on said second capacitance means;

first resistance means coupling said tone signal source to said secondcapacitance means;

a third switch, said third switch being clos able after said secondswitch is opened, and said third switch being operable, when closed, tocouple said second capacitance means and said first resistance means tosaid first capacitance means to thereupon initiate reversal of thecharge of said second capacitance means by said first capacitance andinitiate the negative rise portion of an impulse voltage applied to saidtone signal source;

third capacitance means and second resistance means coupled to saidsecond capacitance means to partially discharge said second capacitancemeans and provide a decrease in the impulse voltage applied to said tonesignal source, said third capacitance means having less capacitance thansaid second capacitance means;

said third and second switches then being operable when again opened andclosed, respectively, to provide a discharge path through said secondswitch to said source of positive potential to rapidly discharge theremaining negative potential from said second capacitance means andreestablish a positive charge thereon.

2. The combination of claim 1 and further comprising:

a fourth switch coupled between to said second switch and said source ofpositive potential and operable, when actuated, to prevent discharge ofsaid second capacitance through said source of positive potential andthereby sustain the application of negative voltage to said tone signalsource.

3. The combination of claim 1 and further comprising:

switch operator means in said coupling means and opening said firstswitch upon the depression of said key to a first extent and closingsaid third switch upon further depression of said key;

and third resistance means coupled in parallel with said firstcapacitance means and providing a discharge path for said firstcapacitance means, to discharge said first capacitance means during theinterval between opening of said first switch and closing of said thirdswitch as said key is depressed.

4. In an electrical musical instrument, the combination comprising:

a plurality of tone signal sources, each having a tone signal outputterminal coupled thereto;

a plurality of note playing keys, each of said keys being coupled to acorresponding one of said output terminals to produce tone signalsselectively at various ones of said output terminals;

a plurality of percussion signal amplitude simulating circuit systems,each of said systems being coupled to one of said keys and to thecorresponding one of said output terminals, and each of said systemscomprising;

a plurality of switches;

means coupling said switches to a key of said plurality of keys wherebythe switches of said plurality are operable in sequence by actuation ofthe y;

first capacitance means coupled through a first switch of said pluralityto a first terminal at substantially constant potential, to charge saidfirst capacitance means;

second capacitance means and first resistance means coupled to one ofsaid tone signal sources, said second capacitance means being coupledthrough a second switch of said plurality of switches to said firstcapacitance means when said second switch is closed to thereuponinitiate charge of said second capacitance means by said firstcapacitance means and initiate the rise portion of an impulse voltageapplied to said one tone signal source;

third capacitance means and second resistance means coupled to saidsecond capacitance means to discharge said second capacitance means andcause a drop of said impulse voltage;

" a third switch coupled between said second capacitance means and asource of potential and operable when closed to rap-idly discharge saidsecond capacitance means through said source of potential.

5. The combination of claim 4 and further comprising:

a fourth switch coupled to said third switch and said source ofpotential and operable, when actuate-d, to prevent discharge of saidsecond capacitance through said source of potential.

6. The combination of claim 4 and further comprising in each of saidsystems:

switch operator means in said coupling means and opening said firstswitch upon the depression of said key to a first extent and closingsaid second switch upon further depression of said key, and thirdresistance means coupled in parallel with said first capacitance meansand providing a discharge path for said first capacitance means, todischarge said first capacitance means during the interval betweenopening of said first switch and closing of said second switch as saidkey is depressed.

7. The combination of claim 4 and further comprising:

a sustain actuator member;

and a sustain switch coupled to said actuator member, said sustainswitch being in the said discharge paths of said systems, and saidsustain switch being operable upon actuation of said actuator member tobreak said discharge paths.

8. A circuit for producing an output simulating the amplitude-timecharacteristic of the sound output of a percussion instrument, saidcircuit comprising:

a first normally-closed switch and a velocity timing circuit coupled inseries across a first source of direct current potential, said velocitytiming circuit including a first resistance and first capacitance;

a second capacitance, a first unidirectional device, a secondresistance, and a second switch connected in series circuit relationshipwith said first capacitance to provide an impulse rise timing circuit,said second switch being normally open;

a third resistance connected in parallel with the series combination ofa fourth resistance and third capacitance, said third resistancebeingconnected in series circuit relationship with said secondcapacitance and a fifth resistance, to provide impulse fall timingcircuit means; i

a normally-closed sustain switch, a normally-closed damper switch, asecond diode, a sixth resistance, and said second capacitance, connectedin series across a second source of direct current potential to providearelease timing circuit;

and a manually operable actuator connected to said switches to firstopen said damper switch and then open said first switch and then closesaid second switch, when said actuator is moved in one direction, andfirst open said second switch and then close said first switch and thenclose said damper switch when said actuator is moved in the oppositedirection, to thereby produce a sharp rise in negative potential appliedto said fifth resistance when said actuator is moved in said firstdirection, followed by a sharp fall in negative potential applied tosaid fifth resistance and then followed by gradual fall of potentialapplied to said fifth resistance, and then followed by a sharp fall ofpotential applied to said fifth resistance when said damper switch isclosed as said actuator is moved in said second direction;

and a sustaining actuator connected to said sustain switch and operable,when actuated, to open said sustain switch to disable said releasetiming circuit and prevent, when said sustain actuator is actuated, thelast mentioned sharp fall of potential applied to said fifth resistance.

9. A percussion tone generating device comprising:

an electronic tone generator means including tone signal output meansfor an electronic musical instrument, said tone generator means havinginput means for a control signal to control amplitude of the outputtone;

and control signal producing circuit means having control voltage outputmeans coupled to said control input means for said tone generator meansand having,

a manual actuator,

switch means coupled to said manual actuator and responsive to operationof said manual actuator,

a velocity timing circuit controlled by said switch means and includinga first charge storage device to establish a potential dependent uponspeed of actuation of said manual actuator,

an impulse forming circuit including a second charge storage device andcontrolled by said switch means to charge said second charge storagedevice according to the charge stored in said velocity timing circuit,said second charge storage device being coupled to said control voltageoutput means,

and a decay timing circuit coupled to said second storage device toaccommodate discharge of said second storage device at a predeterminedrate,

a release timing circuit controlled by said switch means to provide,when desired, a more rapid discharge of said second charge storagedevice than is possible through said decay timing circuit,

and sustain switching means in said release timing circuit toselectively disable said release timing circuit.

10. A percussion tone generating device comprising:

an electronic tone generator means including tone signal output meansfor an electronic musical instrument, said tone generator means havinginput means for a control signal to control amplitude of the outputtone;

and control signal producing circuit means having control voltage outputmeans coupled to said control input means for said tone generator meansand having, a manual actuator.

switch means coupled to said manual actuator and re sponsive tooperation of said manual actuator,

a velocity timing circuit controlled by said switch means and includinga first charge storage device to establish a potential dependent uponspeed of actuation of said manual actuator,

an impulse forming circuit including a second charge storage device andcontrolled by said switch means to charge said second charge storagedevice according to the charge stored in said velocity timing circuit,said second charge storage device being coupled to said control voltageoutput means,

and a decay timing circuit coupled to said second storage device toaccommodate discharge of said second storage device at a predeterminedrate,

and a release timing circuit controlled by said switch means to provide,when desired, a more rapid discharge of said second charge storagedevice than is possible through said decay timing circuit.

11. A circuit responsive to operation of a manual actuator to provide anoutput corresponding to the output of a percussion type of musicalinstrument and comprising:

switching means operable by said actuator;

a first circuit coupled to said switching means and to a source ofpotential and adapted to develop a charge proportional to the speed ofmovement of said actuator;

an impulse initiating circuit coupled through said switching means tosaid first circuit and including a charge storage element, said impulseinitiating circuit being thereby adapted to rapidly charge the saidstorage element therein to a potential determined by the charge attainedby said first timing circuit;

impulse shaping circuit means coupled to said charge storage element ofsaid impulse intiating circuit and comprising means for discharging saidcharge storage element first at a rapid rate and then at a less rapidrate;

and a damper circuit coupled to said charge storage element and saidswitching means to accomplish rapid discharge of said charge storageelement in response to a selected operation of said switching means.

12. In a sound control circuit for an electrical musical instrument, thecombination comprising:

a source of positive direct current potential;

a source of ground potential;

first and second normally closed switches connected in series with afirst diode, a first resistance, and a charge storage element, saidseries connection being connected from said source of positive potentialto said ground, and said second switch being operable by a playing keyof said instrument and said first switch being operable by a sustainpedal of said instrument;

and an output coupled through a resistance to a junction between saidfirst charge storage element and said first resistance.

No references cited.

ARTHUR GAUSS, Primary Examiner. J. BUSCH, Assistant Examiner.

1. IN AN ELECTRICAL MUSICAL INSTRUMENT, THE COMBINATION COMPRISING: ATONE SIGNAL SOURCE HAVING A TONE SIGNAL OUTPUT TERMINAL; A PLURALITY OFKEYS; A PLURALITY OF SWITCHES; MEANS COUPLING SAID PLURALITY OF SWITCHESTO ONE OF SAID KEYS WHEREBY SWITCHES OF SAID PLURALITY ARE OPERABLE INSEQUENCE BY ACTUATION OF ONE OF SAID KEYS; FIRST CAPACITANCE MEANSCOUPLED THROUGH A FIRST SWITCH OF SAID PLURALITY TO A FIRST TERMINAL ATSUBSTANTIALLY CONSTANT NEGATIVE POTENTIAL TO CHARGE SAID FIRSTCAPACITANCE MEANS; SECOND CAPACITANCE MEANS COUPLED THROUGH A SECONDSWITCH OF SAID PLURALITY OF SWITCHES TO A SOURCE OF POSITIVE POTENTIALTO ESTABLISH AN INITIAL POSITIVE CHARGE ON SAID SECOND CAPACITANCEMEANS; FIRST RESISTANCE MEANS COUPLING SAID TONE SIGNAL SOURCE TO SAIDSECOND CAPACITANCE MEANS; A THIRD SWITCH, SAID THIRD SWITCH BEINGCLOSABLE AFTER SAID SECOND SWITCH IS OPENED, AND SAID THIRD SWITCH BEINGOPERABLE, WHEN CLOSED, TO COUPLE SAID SECOND CAPACITANCE MEANS AND SAIDFIRST RESISTANCE MEANS TO SAID FIRST CAPACITANCE MEANS TO THEREUPONINITIATE REVERSAL OF THE CHARGE OF SAID SECOND CAPACITANCE MEANS BY SAIDFIRST CAPACITANCE AND INITIATE THE NEGATIVE RISE PORTION OF AN IMPULSEVOLTAGE APPLIED TO SAID TONE SIGNAL SOURCE; THIRD CAPACITANCE MEANS ANDSECOND RESISTANCE MEANS COUPLED TO SAID SECOND CAPACITANCE MEANS TOPARTIALLY DISCHARGE SAID SECOND CAPACITANCE MEANS AND PROVIDE A DECREASEIN THE IMPULSE VOLTAGE APPLIED TO SAID TONE SIGNAL SOURCE, SAID THIRDCAPACITANCE MEANS HAVING LESS CAPACITANCE THAN SAID SECOND CAPACITANCEMEANS; SAID THIRD AND SECOND SWITCHES THEN BEING OPERABLE WHEN AGAINOPENED AND CLOSED, RESPECTIVELY, TO PROVIDE A DISCHARGE PATH THROUGHSAID SECOND SWITCH TO SAID SOURCE OF POSITIVE POTENTIAL TO RAPIDLYDISCHARGE THE REMAINING NEGATIVE POTENTIAL FROM SAID SECOND CAPACITANCEMEANS AND RE-ESTABLISH A POSITIVE CHARGE THEREON.